When a semiconductor device such as a memory or an integrated circuit is manufactured, in order to avoid the influence due to dust and the like in the air, it is necessary to apply doping and etching to a high-purity semiconductor substrate (wafer) in a chamber in a high vacuum state. A vacuum pump such as a turbo molecular pump is used for exhaust in the chamber.
As such a vacuum pump, there is known a vacuum pump including a thread groove pump mechanism configured by a rotor including an outer cylinder rotor and an inner cylinder rotor, a stator including an outer cylinder stator and an inner cylinder stator alternately positioned between the outer cylinder rotor and the inner cylinder rotor, and thread grooves engraved on a wall surface of the stator opposed to the rotor, wherein gas rises and falls in an S shape in the up-down direction in the thread groove pump mechanism to be exhausted (see, for example, Japanese Patent No. 3961273 (Patent Literature 1)).
As another vacuum pump, there is known a vacuum pump including a substantially cylindrical casing and a thread groove pump mechanism configured by a substantially cylindrical stator disposed in an axial portion of the casing, a rotor, a rotor shaft of which is supported by the axial portion of the stator to be capable of being driven to rotate, the rotor including a substantially cylindrical cylinder portion between the casing and the stator, ridge portions and thread grooves respectively provided on an inner circumferential surface opposed to a cylinder portion of the casing and an outer circumferential surface opposed to a cylinder portion of the stator, wherein gas is exhausted from up to down in the up-down direction in the thread groove pump mechanism (see, for example, Japanese Utility Model Application Publication No. H5-38389 (Patent Literature 2)).
However, in the former vacuum pump explained above, as shown in FIG. 7, gas near an exhaust side outlet 91a of a thread groove 91 of an inner cylinder stator 90 flows into the thread groove 91 forward in a rotating direction R of an inner cylinder rotor 93 climbing over an exhaust side end portion 92a of a ridge portion 92 (a flow of the inflow gas is indicated by an arrow A in FIG. 7). Near the exhaust side outlet 91a of the thread groove 91 into which the gas flows, a flow of the gas tends to be disturbed to cause retention of the gas.
In an exhaust portion of the thread groove pump mechanism, for example, near an upper end face 90a of the inner cylinder stator 90, as indicated by an arrow B in FIG. 8, the gas is sometimes retained while annularly turning along a rotating direction R of the inner cylinder rotor 93 without being sent to an inner circumference side of the inner cylinder stator 90. As indicated by an arrow C in FIG. 8, the gas retained in the exhaust portion flows back to an outer circumference side of the inner cylinder stator 90. Near the exhaust side outlet 91a of the thread groove 91 to which the gas flows back, a flow of the gas tends to be disturbed to cause retention of the gas.
In the former and latter vacuum pumps explained above, on a lower end face of the cylinder portion of the rotor, compressed gas is sometimes retained while annularly turning along a rotor rotating direction. The gas retained while turning sometimes flows back upward in the thread groove pump mechanism and disturbs a flow of the gas in the exhaust side outlet of the thread groove. The gas is sometimes retained in the exhaust side outlet of the thread groove.
When the gas is retained in the exhaust side outlet of the thread groove as explained above, the retained gas solidifies under a high pressure, a gas product is deposited, and a channel of the exhaust side outlet of the thread groove is narrowed. Therefore, it is likely that a compression ratio decreases and pump performance is deteriorated.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.